Understanding Native VLAN Traffic

A

The correct answer is A. Cisco Discovery Protocol (CDP) traffic travels on the native VLAN by default.

The native VLAN is a default VLAN assigned to an 802.1Q trunk port. It carries untagged traffic, which is traffic that does not have a VLAN tag. By default, the native VLAN on Cisco switches is VLAN 1, but this can be changed to any VLAN. The native VLAN is used for communication between switches, and for traffic that is not explicitly assigned to a VLAN.

Option A is correct because Cisco Discovery Protocol (CDP) traffic is sent untagged on the native VLAN by default. CDP is a proprietary protocol developed by Cisco that is used to share information about directly connected Cisco devices, such as the device's hostname, IP address, and capabilities.

Option B is incorrect because traffic on the native VLAN is untagged by definition, so it does not have a VLAN tag. The default VLAN tag for traffic on an 802.1Q trunk port is 1, but this only applies to tagged traffic.

Option C is incorrect because the control plane traffic is not blocked on the native VLAN. Control plane traffic refers to the traffic that is used for the management and control of the network devices, such as routing protocols and management protocols. This traffic can be sent on any VLAN, including the native VLAN.

Option D is also incorrect because the native VLAN is not typically disabled for security reasons. However, it is a common security practice to change the native VLAN to a different VLAN to prevent unauthorized access to the management plane of the switch. This is because the native VLAN is often used for management traffic, and an attacker who gains access to the native VLAN can potentially compromise the switch.